Concrete is a desirable building material because of its durability, cost and ability to withstand enormous compressive forces. Therefore, concrete has typically been used in those applications where a material was needed to accommodate such compressive loads. Examples of such applications are foundations, pillars, sidewalks and freeways.
Although concrete is known for its ability to handle compressive forces it is equally known for its inability to withstand tension forces. Accordingly, concrete is not a popular building material for use in tension applications. However, the cost and durability of concrete as a building material has inspired investigation into the use of fortified concrete structures in tension service. It was discovered that a concrete structure could be used in a tension service if an external compressive force was applied so that the structure is subjected to a net overall compression force. This method of applying an external compressive force to the concrete structure is called prestressing.
A popular method for prestressing concrete used in tension service is to wrap the concrete structure with high-strength wire under sufficient tension to achieve a net overall compression force on the structure when in service. A popular use of such prestressed concrete is in the formation of concrete pipes. Concrete piping is commonly used in those applications where the cost of alternative materials render their use prohibitive. Examples of such uses include large water mains, dams or other fluid transport systems that are characterized by the large volumes of fluid that must be transported at appreciable internal pressure. Accordingly, the diameter of piping necessary to transport such volumes range from about three feet up to about 22 feet in some applications. Concrete is the most economic building material in these applications due to the amount of material necessary to manufacture such large diameter pipes.
Concrete pipe must be prestressed because its inner diameter will be subjected to the internal hydraulic pressures required for the transport of fluid. This hydraulic pressure exerts a tension force uniformly about the inside diameter of the concrete pipe. In order to keep a net compression load on the pipe while its in service the pipe is wrapped in a continuous spiral of wire subjected to a tension sufficient to overcome the applied internal hydraulic pressure. Accordingly, to insure that the concrete pipe is in net compression it is of extreme importance that the wire wrapped around the concrete pipe be maintained in tension within a precise tolerance range at all times.
After applying and anchoring the prestressing wire, the entire pipe is coated with a concrete mortar in sufficient thickness to embed the wire and protect it from the environment the pipe will encounter in service.
In such applications, it may occur that the wire wrapped around the pipe becomes corroded and may eventually fail. Corrosion may occur, for example, if the mortar coating is broken or cracked. Additionally, in order to determine the condition of such concrete pipes it is often desirable to remove a section of the prestressing wire and inspect the wire for signs of abnormal stress or metallurgical defects. The result of a wire failure due to corrosion or the removal of a section of wire for inspection purposes is the sudden loss of part of the applied compressive force upon the pipe. The loss of the applied compressive force provided by the wire subjects the concrete pipe to the net overall tension force from the internal hydraulic pressure, which may result in the catastrophic failure of the pipe.
Previous efforts to replace wire because of corrosion or for metallurgical testing have been unsuitable. Wire has been cut out and a splice wire welded in place to the remaining ends. The welding not only damages the metallurgical properties of the wire, the spliced piece is not stressed and there may be insufficient prestress in the concrete in the region of the splice.
It is therefore, highly desirable to provide a means for restoring the compressive force applied to the concrete pipe that is necessary to overcome the internal hydraulic pressure after a failure or removal of the prestressing wire has occurred. It is also desirable that this means allow for the application of a known amount of compression upon the concrete pipe and that the means be relatively easy to use.
Ordinary means for stressing the wire when a section is replaced are not suitable since proper prestressing of a cylindrical object such as a pipe requires that the wire fit snugly against the surface of the concrete. Other means for holding and stressing the wire would raise the wire from the pipe surface.